The invention relates to a circuit configuration for programming an electrically programmable element.
Integrated circuits often have redundancy circuits for repairing defective circuit sections. In particular in the case of integrated memory circuits, these circuits may be, for example, regular word or bit lines with defective memory cells which are replaced by redundant word or bit lines. To that end, the integrated memory is tested for example by a self-test device and a programming of the redundant elements is subsequently performed. A redundancy circuit then has programmable elements, e.g. in the form of electrical fuses, which serve for storing the address of a line to be replaced. The electrical fuses are electrical connection elements whose conductor track resistance can be permanently altered. They can be programmed at the end of the process for fabricating the integrated circuit by applying a so-called burning voltage.
For programming purposes, a burning voltage having a high potential level is usually applied to the circuit externally. The operation for programming electrically programmable elements is then effected for example through the use of a high current, which leads to a lasting alteration of the conductor track resistance, for example by causing a corresponding electrical fuse to melt.
It has been shown that the programming or burning operation which is carried out in order to program programmable elements is not always effected reliably. Moreover, in the course of programming a plurality of programmable elements, the time duration required for the burning operation of one of the programmable elements may differ from the time durations required for the other programmable elements. This may result e.g. from manufacturing tolerances present in the programmable elements. The time durations of the burning operations are usually chosen uniformly for all the elements to be programmed and are preset for example by an internal control of the integrated circuit. Consequently, given a predetermined, uniform time duration of all the programming operations to be performed, the time duration should be chosen relatively generously in order that as far as possible all the programmable elements will be completely programmed. However, the consequence of this is that the test time of an integrated circuit with the subsequent programming of a multiplicity of programmable elements can increase considerably.
If no monitoring or check with regard to whether the programming of the programmable elements has been carried out successfully, is performed at the end of this process, then an unnoticed quality defect of the relevant integrated circuit can result if a defectively programmed element is present.
To date, it has been customary to perform all the necessary programming or burning operations of the elements to be programmed and subsequently to monitor them. Once such test results are available, a decision can be taken as to whether the relevant circuit will be subjected to another burning operation. The consequence of this is that the time duration for the completion of the integrated circuit is prolonged once again. If, on the other hand, a further burning operation is dispensed with, the integrated circuit must be identified as defective, for example. This means that, in the course of the fabrication of a plurality of integrated circuits, the yield of integrated circuits having entirely satisfactory quality decreases.
It is accordingly an object of the invention to provide a circuit configuration for programming an electrically programmable element which overcomes the above-mentioned disadvantages of the heretofore-known circuit configurations of this general type and which allows to perform a monitoring of the burning operation of the electrically programmable element during the programming operation.
With the foregoing and other objects in view there is provided, in accordance with the invention, a circuit configuration for programming an electrically programmable element, including:
a programmable element having an input, an output, and a conductor track resistance between the input and the output, the conductor track resistance being permanently alterable by one of an electric current and an electric voltage;
a switchable element having a controlled path and a control terminal for providing a control signal for programming the programmable element;
a first contact point for a first supply potential and a second contact point for a second supply potential;
the controlled path of the switchable element and the programmable element being connected in series for forming a series circuit, the series circuit being connected between the first contact point for the first supply potential and the second contact point for the second supply potential;
the programmable element being connected to the first contact point for the first supply potential, the switchable element being connected to the second contact point for the second supply potential; and
a monitoring circuit connected in series to the controlled path of the switchable element and the programmable element between the first contact point for the first supply potential and the second contact point for the second supply potential, the monitoring circuit configured for measuring an electrical parameter characteristic for a programming operation.
The circuit configuration has a programmable element, whose conductor track resistance can be permanently altered by an electric current or an electric voltage, and also a switchable element, which has a control terminal with a control signal for programming the programmable element. The programmable element and the switchable element are connected in series between two supply potentials. In this case, the programmable element is connected to a terminal for a first supply potential, and the switchable element is connected to a terminal for a second supply potential. During a burning operation of the electrically programmable element, the first supply potential assumes the value of a burning voltage, for example, and the second supply potential the value of a reference-ground voltage. A monitoring circuit is connected in series with this series circuit between the terminals for the first and the second supply potential, for the purpose of measuring an electrical characteristic quantity or parameter which is characteristic of the programming operation. In this case, a characteristic quantity is used which provides information about whether or when a burning operation has been concluded.
This makes it possible for the integrated circuit to monitor the burning operation of the element to be programmed itself. Using this monitoring information, a control process which automatically controls the programming operation can be integrated in the programming operation. This can be realized for example by a control circuit which is integrated on the chip, evaluates the measurements of the monitoring circuit step by step, and uses this information to monitor or control the burning operation. In this way, it is possible to intervene in a programming operation already during the programming operation. Compared to the preset time duration, the programming operation can be correspondingly lengthened, so that it is terminated, as far as the time is concerned, and a renewed programming operation does not have to be initiated. Likewise, the programming operation can be automatically shortened as soon as it is completely terminated. In both cases, a minimum time duration which is necessary for completely programming a programmable element is ensured.
Furthermore, the circuit configuration according to the invention may advantageously be used in an integrated circuit containing a self-test device with a self-repair function. By virtue of an automatic control of the programming operation, the quality of a self-repair that has been performed is increased in a circuit of this type.
In a preferred embodiment of the invention, the electric current of the series circuit between the terminals for the first and second supply potentials is used as the measured characteristic quantity. As a result of the potential difference, a current flows in the series circuit formed by the programmable element and the switchable element. The value of this current likewise changes on account of the change of the conductor track resistance of the programmable element. In the course of a burning operation, this burning current alters significantly, and abruptly in the case of an electrical fuse. As soon as this alteration has occurred, it can be assumed that the electrical fuse is completely programmed.
A simple embodiment of the monitoring circuit for measuring the burning current provides a current mirror which is connected with its input path in series with the series circuit formed by the programmable element and the switchable element. A terminal for an output signal of the monitoring circuit, which is used for evaluation purposes, is connected to the output path of the current mirror. The use of a current mirror ensures a complete decoupling of the burning current of the series circuit formed by the programmable element and the switchable element and of the measurement current in the output path of the current mirror. Moreover, the profile of the burning current can be reconstructed in a simple manner by virtue of the proportional relationship between the burning current in the input path of the current mirror and the measurement current in the output path of the current mirror.
In a further embodiment, the voltage potential at that terminal of the programmable element which is remote from the first supply potential is used as the measured characteristic quantity. Its value is dependent on the size of the conductor track resistance of the programmable element and of the contact resistance of the switchable element in conjunction with the magnitude of the burning current, and likewise alters in the course of the programming. This embodiment has the advantage over direct measurement of the burning current that it is not necessary to provide a further component in the series circuit formed by the programmable element and the switchable element. On account of the high potential differences between the first and second supply potentials, the component must be dimensioned such that it is relatively large, resulting in an increased space requirement if there are a multiplicity of programmable elements.
This embodiment is realized for example by a monitoring circuit which once again has a current mirror which is connected by a first terminal of the input path to that terminal of the programmable element which is remote from the first supply potential, and by a second terminal of the input path to the terminal for the second supply potential. The terminal for the output signal of the monitoring circuit is connected to the output path of the current mirror. The current which is present, proportionally to the potential difference, in the input path is mirrored into the output path and can be measured using the output signal of the monitoring circuit.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a circuit configuration for programming an electrically programmable element, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.